signals, and makes a permanent strip chart or digital record. In vessels with 

 a continuing need for a Fathometer, the transducer is usually permanently 

 mounted in the hull. For temporary use, the transducer is usually mounted 

 outboard and secured to a rail or bracket. 



75. Modern Fathometers have a range of capabilities and features that 

 allows manual or automatic selection of various parameters such as chart speed 

 and scale. For borrow source exploration programs, features considered neces- 

 sary are event markers and a means of selecting the chart scale, chart speed, 

 and correction for transducer depth. Automatic event marking and annotation 

 are valuable options that can save on personnel costs. Many available Fath- 

 ometers contain other features that improve quality and operational capabili- 

 ties but are not essential. These features provide automatic corrections for 

 tidal stage, variations in sound velocity, and vessel motions. Other features 

 allow selective adjustment for acoustic pulse frequency, energy, pulse length 

 and repetition rate, and automatic adjustment for sensitivity. 



76. A recent development that would be valuable in borrow source explo- 

 ration is a signal processing unit that can be interfaced with a Fathometer 

 and used to indicate the sea floor sediments in terms of Wentworth or other 

 general classification systems. This is accomplished by measurements of two 

 independent variables, roughness and hardness, from the acoustic signal and 

 interpreting these data in terms of sediment type. 



Seismic Reflection Systems 



77. Seismic reflection systems are similar to Fathometers in their 

 basic principles of operation; however, they normally use acoustic signals of 

 lower frequency and higher energy. These pulses are not totally reflected or 

 attenuated at the bottom-water interface, but part of the sound energy pene- 

 trates the sub -bottom zone where it reflects from interfaces separating rock 

 or sediment units having different acoustic impedance properties (Figure 7). 

 The greater the difference, the greater the reflection of acoustic energy. 

 The impedance of a medium is the product of its bulk density and the velocity 

 of a compressional wave traveling through it. The compressional wave velocity 

 is in turn a function of the bulk density, compressibility, and rigidity of 

 the material. These factors are usually related to lithology. Consequently, 

 seismic reflection profiles can be roughly analogous to a geological cross 



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